For both copies of a duplicated gene to become fixed in a population and subsequently maintained, selection must favour individuals with both genes over individuals with one. Here I review and assess some of the proposed ways that gene structure and function might affect the likelihood of both copies acquiring distinct functions and therefore positive selection. In particular I focus on the interacting pathways of genes that make up gene networks, and how these may affect genes duplicated both singly and en masse. Using the Wnt and hedgehog pathways as examples and data from developmental and genome analyses, I show that, while some of these theories may genuinely reflect what has occurred in animal evolution, there are still insufficient data to rigorously assess their relative importance. This, however, is likely to change in the near future.